Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

From: "Paul E. McKenney" <[email-masked]>
Date: Sat, 25 Oct 2014 06:38:20 -0700

On Sat, Oct 25, 2014 at 03:09:36PM +0300, Yanko Kaneti wrote:
> On Fri-10/24/14-2014 14:49, Paul E. McKenney wrote:
> > On Sat, Oct 25, 2014 at 12:25:57AM +0300, Yanko Kaneti wrote:
> > > On Fri-10/24/14-2014 11:32, Paul E. McKenney wrote:
> > > > On Fri, Oct 24, 2014 at 08:35:26PM +0300, Yanko Kaneti wrote:
> > > > > On Fri-10/24/14-2014 10:20, Paul E. McKenney wrote:
> > 
> > [ . . . ]
> > 
> > > > > > Well, if you are feeling aggressive, give the following patch a spin.
> > > > > > I am doing sanity tests on it in the meantime.
> > > > > 
> > > > > Doesn't seem to make a difference here
> > > > 
> > > > OK, inspection isn't cutting it, so time for tracing.  Does the system
> > > > respond to user input?  If so, please enable rcu:rcu_barrier ftrace before
> > > > the problem occurs, then dump the trace buffer after the problem occurs.
> > > 
> > > Sorry for being unresposive here, but I know next to nothing about tracing
> > > or most things about the kernel, so I have some cathing up to do.
> > > 
> > > In the meantime some layman observations while I tried to find what exactly
> > > triggers the problem.
> > > - Even in runlevel 1 I can reliably trigger the problem by starting libvirtd
> > > - libvirtd seems to be very active in using all sorts of kernel facilities
> > >   that are modules on fedora so it seems to cause many simultaneous kworker 
> > >   calls to modprobe
> > > - there are 8 kworker/u16 from 0 to 7
> > > - one of these kworkers always deadlocks, while there appear to be two
> > >   kworker/u16:6 - the seventh
> > 
> > Adding Tejun on CC in case this duplication of kworker/u16:6 is important.
> > 
> > >   6 vs 8 as in 6 rcuos where before they were always 8
> > > 
> > > Just observations from someone who still doesn't know what the u16
> > > kworkers are..
> > 
> > Could you please run the following diagnostic patch?  This will help
> > me see if I have managed to miswire the rcuo kthreads.  It should
> > print some information at task-hang time.
> 
> So here the output with todays linux tip and the diagnostic patch.
> This is the case with just starting libvird in runlevel 1.

Thank you for testing this!

> Also a snapshot  of the kworker/u16 s
> 
>     6 ?        S      0:00  \_ [kworker/u16:0]
>   553 ?        S      0:00  |   \_ [kworker/u16:0]
>   554 ?        D      0:00  |       \_ /sbin/modprobe -q -- bridge
>    78 ?        S      0:00  \_ [kworker/u16:1]
>    92 ?        S      0:00  \_ [kworker/u16:2]
>    93 ?        S      0:00  \_ [kworker/u16:3]
>    94 ?        S      0:00  \_ [kworker/u16:4]
>    95 ?        S      0:00  \_ [kworker/u16:5]
>    96 ?        D      0:00  \_ [kworker/u16:6]
>   105 ?        S      0:00  \_ [kworker/u16:7]
>   108 ?        S      0:00  \_ [kworker/u16:8]

You had six CPUs, IIRC, so the last two kworker/u16 kthreads are surplus
to requirements.  Not sure if they are causing any trouble, though.

> INFO: task kworker/u16:6:96 blocked for more than 120 seconds.
>       Not tainted 3.18.0-rc1+ #16
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> kworker/u16:6   D ffff8800ca9ecec0 11552    96      2 0x00000000
> Workqueue: netns cleanup_net
>  ffff880221fff9c8 0000000000000096 ffff8800ca9ecec0 00000000001d5f00
>  ffff880221ffffd8 00000000001d5f00 ffff880223260000 ffff8800ca9ecec0
>  ffffffff82c44010 7fffffffffffffff ffffffff81ee3798 ffffffff81ee3790
> Call Trace:
>  [<ffffffff81866219>] schedule+0x29/0x70
>  [<ffffffff8186b43c>] schedule_timeout+0x26c/0x410
>  [<ffffffff81028bea>] ? native_sched_clock+0x2a/0xa0
>  [<ffffffff8110748c>] ? mark_held_locks+0x7c/0xb0
>  [<ffffffff8186c4c0>] ? _raw_spin_unlock_irq+0x30/0x50
>  [<ffffffff8110761d>] ? trace_hardirqs_on_caller+0x15d/0x200
>  [<ffffffff81867c4c>] wait_for_completion+0x10c/0x150
>  [<ffffffff810e4dc0>] ? wake_up_state+0x20/0x20
>  [<ffffffff81133627>] _rcu_barrier+0x677/0xcd0
>  [<ffffffff81133cd5>] rcu_barrier+0x15/0x20
>  [<ffffffff81720edf>] netdev_run_todo+0x6f/0x310
>  [<ffffffff81715aa5>] ? rollback_registered_many+0x265/0x2e0
>  [<ffffffff8172df4e>] rtnl_unlock+0xe/0x10
>  [<ffffffff81717906>] default_device_exit_batch+0x156/0x180
>  [<ffffffff810fd280>] ? abort_exclusive_wait+0xb0/0xb0
>  [<ffffffff8170f9b3>] ops_exit_list.isra.1+0x53/0x60
>  [<ffffffff81710560>] cleanup_net+0x100/0x1f0
>  [<ffffffff810cc988>] process_one_work+0x218/0x850
>  [<ffffffff810cc8ef>] ? process_one_work+0x17f/0x850
>  [<ffffffff810cd0a7>] ? worker_thread+0xe7/0x4a0
>  [<ffffffff810cd02b>] worker_thread+0x6b/0x4a0
>  [<ffffffff810ccfc0>] ? process_one_work+0x850/0x850
>  [<ffffffff810d337b>] kthread+0x10b/0x130
>  [<ffffffff81028c69>] ? sched_clock+0x9/0x10
>  [<ffffffff810d3270>] ? kthread_create_on_node+0x250/0x250
>  [<ffffffff8186d1fc>] ret_from_fork+0x7c/0xb0
>  [<ffffffff810d3270>] ? kthread_create_on_node+0x250/0x250
> 4 locks held by kworker/u16:6/96:
>  #0:  ("%s""netns"){.+.+.+}, at: [<ffffffff810cc8ef>]
>  #process_one_work+0x17f/0x850
>  #1:  (net_cleanup_work){+.+.+.}, at: [<ffffffff810cc8ef>]
>  #process_one_work+0x17f/0x850
>  #2:  (net_mutex){+.+.+.}, at: [<ffffffff817104ec>] cleanup_net+0x8c/0x1f0
>  #3:  (rcu_sched_state.barrier_mutex){+.+...}, at: [<ffffffff81133025>]
>  #_rcu_barrier+0x75/0xcd0
> rcu_show_nocb_setup(): rcu_sched nocb state:
>   0: ffff8802267ced40 l:ffff8802267ced40 n:ffff8802269ced40 .G.
>   1: ffff8802269ced40 l:ffff8802267ced40 n:          (null) ...
>   2: ffff880226bced40 l:ffff880226bced40 n:ffff880226dced40 .G.
>   3: ffff880226dced40 l:ffff880226bced40 n:          (null) N..
>   4: ffff880226fced40 l:ffff880226fced40 n:ffff8802271ced40 .G.
>   5: ffff8802271ced40 l:ffff880226fced40 n:          (null) ...
>   6: ffff8802273ced40 l:ffff8802273ced40 n:ffff8802275ced40 N..
>   7: ffff8802275ced40 l:ffff8802273ced40 n:          (null) N..

And this looks like rcu_barrier() has posted callbacks for the
non-existent CPUs 7 and 8, similar to what Jay was seeing.

I am working on a fix -- chasing down corner cases.

							Thanx, Paul

> rcu_show_nocb_setup(): rcu_bh nocb state:
>   0: ffff8802267ceac0 l:ffff8802267ceac0 n:ffff8802269ceac0 ...
>   1: ffff8802269ceac0 l:ffff8802267ceac0 n:          (null) ...
>   2: ffff880226bceac0 l:ffff880226bceac0 n:ffff880226dceac0 ...
>   3: ffff880226dceac0 l:ffff880226bceac0 n:          (null) ...
>   4: ffff880226fceac0 l:ffff880226fceac0 n:ffff8802271ceac0 ...
>   5: ffff8802271ceac0 l:ffff880226fceac0 n:          (null) ...
>   6: ffff8802273ceac0 l:ffff8802273ceac0 n:ffff8802275ceac0 ...
>   7: ffff8802275ceac0 l:ffff8802273ceac0 n:          (null) ...
> INFO: task modprobe:554 blocked for more than 120 seconds.
>       Not tainted 3.18.0-rc1+ #16
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> modprobe        D ffff8800c85dcec0 12456   554    553 0x00000000
>  ffff8802178afbf8 0000000000000096 ffff8800c85dcec0 00000000001d5f00
>  ffff8802178affd8 00000000001d5f00 ffffffff81e1b580 ffff8800c85dcec0
>  ffff8800c85dcec0 ffffffff81f90c08 0000000000000246 ffff8800c85dcec0
> Call Trace:
>  [<ffffffff818667c1>] schedule_preempt_disabled+0x31/0x80
>  [<ffffffff81868013>] mutex_lock_nested+0x183/0x440
>  [<ffffffff8171037f>] ? register_pernet_subsys+0x1f/0x50
>  [<ffffffff8171037f>] ? register_pernet_subsys+0x1f/0x50
>  [<ffffffffa0619000>] ? 0xffffffffa0619000
>  [<ffffffff8171037f>] register_pernet_subsys+0x1f/0x50
>  [<ffffffffa0619048>] br_init+0x48/0xd3 [bridge]
>  [<ffffffff81002148>] do_one_initcall+0xd8/0x210
>  [<ffffffff8115bc22>] load_module+0x20c2/0x2870
>  [<ffffffff81156c00>] ? store_uevent+0x70/0x70
>  [<ffffffff81281327>] ? kernel_read+0x57/0x90
>  [<ffffffff8115c5b6>] SyS_finit_module+0xa6/0xe0
>  [<ffffffff8186d2d5>] ? sysret_check+0x22/0x5d
>  [<ffffffff8186d2a9>] system_call_fastpath+0x12/0x17
> 1 lock held by modprobe/554:
>  #0:  (net_mutex){+.+.+.}, at: [<ffffffff8171037f>]
>  #register_pernet_subsys+0x1f/0x50
> rcu_show_nocb_setup(): rcu_sched nocb state:
>   0: ffff8802267ced40 l:ffff8802267ced40 n:ffff8802269ced40 .G.
>   1: ffff8802269ced40 l:ffff8802267ced40 n:          (null) ...
>   2: ffff880226bced40 l:ffff880226bced40 n:ffff880226dced40 .G.
>   3: ffff880226dced40 l:ffff880226bced40 n:          (null) N..
>   4: ffff880226fced40 l:ffff880226fced40 n:ffff8802271ced40 .G.
>   5: ffff8802271ced40 l:ffff880226fced40 n:          (null) ...
>   6: ffff8802273ced40 l:ffff8802273ced40 n:ffff8802275ced40 N..
>   7: ffff8802275ced40 l:ffff8802273ced40 n:          (null) N..
> rcu_show_nocb_setup(): rcu_bh nocb state:
>   0: ffff8802267ceac0 l:ffff8802267ceac0 n:ffff8802269ceac0 ...
>   1: ffff8802269ceac0 l:ffff8802267ceac0 n:          (null) ...
>   2: ffff880226bceac0 l:ffff880226bceac0 n:ffff880226dceac0 ...
>   3: ffff880226dceac0 l:ffff880226bceac0 n:          (null) ...
>   4: ffff880226fceac0 l:ffff880226fceac0 n:ffff8802271ceac0 ...
>   5: ffff8802271ceac0 l:ffff880226fceac0 n:          (null) ...
>   6: ffff8802273ceac0 l:ffff8802273ceac0 n:ffff8802275ceac0 ...
>   7: ffff8802275ceac0 l:ffff8802273ceac0 n:          (null) ...
> 
> 
> 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > rcu: Dump no-CBs CPU state at task-hung time
> > 
> > Strictly diagnostic commit for rcu_barrier() hang.  Not for inclusion.
> > 
> > Signed-off-by: Paul E. McKenney <[email-masked]>
> > 
> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index 0e5366200154..34048140577b 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -157,4 +157,8 @@ static inline bool rcu_is_watching(void)
> >  
> >  #endif /* #else defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */
> >  
> > +static inline void rcu_show_nocb_setup(void)
> > +{
> > +}
> > +
> >  #endif /* __LINUX_RCUTINY_H */
> > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> > index 52953790dcca..0b813bdb971b 100644
> > --- a/include/linux/rcutree.h
> > +++ b/include/linux/rcutree.h
> > @@ -97,4 +97,6 @@ extern int rcu_scheduler_active __read_mostly;
> >  
> >  bool rcu_is_watching(void);
> >  
> > +void rcu_show_nocb_setup(void);
> > +
> >  #endif /* __LINUX_RCUTREE_H */
> > diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> > index 06db12434d72..e6e4d0f6b063 100644
> > --- a/kernel/hung_task.c
> > +++ b/kernel/hung_task.c
> > @@ -118,6 +118,7 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
> >  		" disables this message.\n");
> >  	sched_show_task(t);
> >  	debug_show_held_locks(t);
> > +	rcu_show_nocb_setup();
> >  
> >  	touch_nmi_watchdog();
> >  
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 240fa9094f83..6b373e79ce0e 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -1513,6 +1513,7 @@ rcu_torture_cleanup(void)
> >  {
> >  	int i;
> >  
> > +	rcu_show_nocb_setup();
> >  	rcutorture_record_test_transition();
> >  	if (torture_cleanup_begin()) {
> >  		if (cur_ops->cb_barrier != NULL)
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 927c17b081c7..285b3f6fb229 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2699,6 +2699,31 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
> >  
> >  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> >  
> > +void rcu_show_nocb_setup(void)
> > +{
> > +#ifdef CONFIG_RCU_NOCB_CPU
> > +	int cpu;
> > +	struct rcu_data *rdp;
> > +	struct rcu_state *rsp;
> > +
> > +	for_each_rcu_flavor(rsp) {
> > +		pr_alert("rcu_show_nocb_setup(): %s nocb state:\n", rsp->name);
> > +		for_each_possible_cpu(cpu) {
> > +			if (!rcu_is_nocb_cpu(cpu))
> > +				continue;
> > +			rdp = per_cpu_ptr(rsp->rda, cpu);
> > +			pr_alert("%3d: %p l:%p n:%p %c%c%c\n",
> > +				 cpu,
> > +				 rdp, rdp->nocb_leader, rdp->nocb_next_follower,
> > +				 ".N"[!!rdp->nocb_head],
> > +				 ".G"[!!rdp->nocb_gp_head],
> > +				 ".F"[!!rdp->nocb_follower_head]);
> > +		}
> > +	}
> > +#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_show_nocb_setup);
> > +
> >  /*
> >   * An adaptive-ticks CPU can potentially execute in kernel mode for an
> >   * arbitrarily long period of time with the scheduling-clock tick turned
> > 
>